For a number of years, it has been known to manufacture building panels for wall and roof construction by using the so-called sandwich principle, whereby at least one surface of a layer or lamina of thermal insulating material, such as cellular polystyrene or polyurethane, is adhered by glue to a metal layer or sheet, such as a galvanized steel sheet having a plastic coated outer surface. As a result of this configuration, the metal layer of such prior art panels functions as a weight carrying part of the completed panel. This type of construction provides several advantages with regard to both the manufacturing technique and the ultimate use of the panel. Particularly, the method for manufacturing such panels is simple and the panels themselves have both weight carrying and thermal insulating properties.
Such prior art panels, however, have the disadvantage that they cannot be classified among those construction materials which are considered to have fireproof properties. A fireproof construction panel must have such properties that it can maintain its weight carrying capacity for an extended period of time in spite of heat acting directly on the panel during a fire. Prior art sandwich-type panels having a core made of a layer of expanded polystyrene foam with steel sheets glued to both surfaces of the core, do not fulfil the fireproof requirements which have been established by many governmental authorities. For example, Finland has its own requirements for fireproof building materials which state that if a certain building panel is to be classified as fireproof in the Class B-10, it must endure heat for ten minutes and maintain its weight carrying capacity during that time.
Prior art building panels very quickly loose their weight carrying capacity after they have been exposed to heat. It has been observed that shortly after the surface of such prior art panels has been exposed to heat, the exposed steel layer buckles away from the core due to the thermal expansion and degradation of the laminating glue caused by heat. As a result, the prior art panel looses a substantial part of its weight carrying capacity. Another important feature of prior art building panels which greatly affects their fireproof properties is that the temperature at which polystyrene softens and becomes gaseous is approximately 100 degrees Celsius. This means that if the heat acting on the surface of the panel causes the temperature inside the panel to rise above 100 degrees Celsius, the polystyrene core starts very quickly to become gaseous. Because the polystyrene core is the layer of the panel which joins the two outer steel layers to each other, the result of gasification of the core is almost instantaneous loss of weight carrying capacity and total collapse of the panel.